High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

Creep tests were performed on MC2 single crystal superalloy at 950 degrees C/200MPa and 1150 degrees C/80MPa under isothermal and thermal cycling conditions with a tensile axis along the [0 0 1] direction. It was found that the thermal cycles strongly affect the creep behavior at 1150 degrees C but not at 950 degrees C. This was related to the repetitive precipitation and dissolution of small gamma' rafts at the higher temperature, as revealed by quantitative characterization of the gamma/gamma' microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[1 0 0]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appears that the density of a[1 0 0] dislocations, and not their climb velocity or diffusion rate, is the key parameter for the control of creep rate. The thermal cycles, which imply the creation and subsequent dissolution of rafts, provided new dislocations, which explains the acceleration of creep observed under such conditions.